SunTrust Banks and Capital One Financial experienced intermittent disruption of their websites Wednesday and Tuesday, respectively, during distributed denial of service (DDoS) attacks, representatives from the banks said via email.

Capital One said access to online banking and mobile services was slow or impossible for a few hours, with full service restored by early afternoon. SunTrust provided fewer details, but confirmed its site experienced problems.

"We have seen increased online traffic today, and experienced intermittent service availability of some of our online functions," a spokesman said.

The hacktivist group, which calls itself Izz ad-Din al-Qassam Cyber Fighters, said in a Pastebin post that it would launch attacks Tuesday, Wednesday and Thursday against Capital One, SunTrust Banks and Regions Financial, respectively.

Regions Financial told Fox Business it was aware of the threat and was preparing its defenses. "We take online security seriously and are taking every measure to protect the company and our customers," the bank said.

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During the last two weeks of September, the self-described Islamic group claimed responsibility for DDoS attacks against PNC Bank, Wells Fargo, US Bank, Bank of America and JPMorgan Chase. The group claims it is retaliating against a video trailer denigrating the Prophet Muhammad. The amateurish video discovered on YouTube last month sparked violent protests in the Middle East and other regions.

Given the sophistication of the DDoS attacks, experts believe the attackers are a well-organised group with significant resources. The attacks have included flooding bank Web servers with use amounts of junk data, while also targeting web applications the hackers deemed vulnerable. No data has been stolen, say the banks.

On Wednesday, Andy Ellis, chief security officer for Akamai Technologies, said Capital One's site was serving an error page during that attack that indicated an application and database overload on the Web server.

Dan Holden, director of research at Arbor Networks, said the attackers were compromising PHP applications on web servers and Wordpress sites using the out-of-date TimThumb plugin in order to deploy tools that allowed them to control the attacks from the hijacked servers. (PHP is a general-purpose server-side scripting language and TimThumb is a PHP script used to resize images.)

"Attackers connect to the tools directly or through intermediate servers/proxies/scripts and therefore the concept of command and control does not apply in the usual manner," Holden said in an email.

In more typical cyberattacks, criminals set up command and control servers in advance in order to control large networks of hijacked personal computers, called botnets.

While the sophistication of the bank attacks indicates the perpetrators are more than a ragtag group of hacktivists, security experts have not been able to pinpoint their location.